Growing Bonsai From Air Layering
Air layering has probably been practiced for the last 1500 years and is still widely used. The ancient Chinese might have discovered the process purely by chance: for example, by noticing a tree or branch which had partially snapped and then rooted itself in the ground.
In the hot and humid conditions of the tropics, air layering is a quick, and therefore frequent, process; in India, for instance, many varieties of fruit tree, such as mango and guava, are propagated by air layerings. The usual method is to wrap a lump of wet clay around a partially severed branch. The ball of clay is then tied on with burlap sacking, and in a matter of weeks a mass of roots will have formed.
The main advantage of air layering is that it results in a mature tree, which can produce fruit from an early age, in a very short space of time. This, of course, makes it an ideal method for bonsai, as it enables you to produce a mature tree in a fraction of the time it takes to grow a tree of similar trunk thickness from seed or a cutting. In addition, it enables you to select a branch which has the most desirable shape and character.
It would seem surprising, therefore, that despite its many attractions, air layering is not used today as a commercially viable method of bonsai production. The reason usually given by nurseries is that air layering is too labor-intensive. As in the case of material collected from the wild, the supply is to a large extent limited and, once exhausted, it will be a long time before more material of the same quality is available. Where the amateur bonsai enthusiast is concerned, however, air layering is an ideal method of propagation as it is simple, cheap and quick.
The technique of air layering is based on deliberately interrupting the flow of sap to a branch. When this happens, the branch will fight to survive; either by bridging the restriction, or by sending out new roots to draw moisture and nutrients from its immediate environment.
In essence, there are two basic methods of interrupting the flow of sap to the branch; however, there are also many variations on these. In the first method you cut a ring of bark from around the branch or trunk, while in the second you apply a tourniquet around the branch so that sap cannot flow through the bark. Because the process of interrupting the sap flow is such a traumatic one, it is often advisable to leave a tiny sliver of bark to act as a bridge, or feeder, so that the end of the branch will continue to obtain nourishment, albeit at a much slower rate.
Matching method to tree
Your choice of a suitable method of air layering will depend largely on the variety of tree: for example, some varieties respond well to complete removal of a ring of bark from the branch, while others may find the shock too great and will die as a result. The complete bark removal is most successful with the following varieties: Japanese maple, trident maple, Chinese elm, zelkova, all the junipers, willows, and cotoneaster.
Air layering is particularly suited to very tall nursery trees which are usually grown as standards. These are trees which have had all their shoots removed up to a height of 1.8m (6ft), leaving a "head" of branches growing from a long stem. A typical 1.8m (6ft) tree could produce as many as nine or ten trees from just one stock plant by air layering it from top to bottom. By selecting the varieties of tree which air layer easily, and by air layering a couple of sections at a time you can produce as many as six or seven all in the space of one growing season.
Deciduous and evergreen trees
As a general rule, deciduous trees respond best to air layering methods which involve the complete removal of bark, while evergreen conifers respond better to either the bridge method, or the wire tourniquet method. The only exception to this rule is the juniper. Almost all the junipers layer very readily when the complete ring-bark technique is used.
The amount of time between starting the air layering and the appearance of roots varies according to the species of tree. Junipers have been known to produce roots in as little as two weeks; pines, on the other hand, are notoriously slow. White pines can take one, or even two, years to send out roots which will adequately sustain the new tree. With pines, the wire tourniquet method is probably best, as the complete ring bark method is too drastic. You can also use the sliver variant of the ring bark method, or the multiple bridge method.
Successful air layering
Air layerings are best taken in the early part of the growing season (i.e. early spring), when the sap is beginning to rise strongly. The other advantage of starting early is that you can air layer continuously from a single tree from early spring to early fall.
Some practitioners recommend wrapping the ball of sphagnum moss with clear plastic, followed by a further layer of black plastic sheeting. However the black plastic is unnecessary as the sphagnum moss itself will exclude much of the light from around the area of trunk or branch that is being air layered. The advantage of only using clear plastic is that you will be able to see the roots once they have formed and come through the sphagnum moss. You will, therefore, know precisely when the branch can be cut off.
The point at which the branch is severed from the parent tree is absolutely critical for successful air layerings. If it is cut off too soon, the air layering will not survive. The branch being air layered should only be cut off when sufficient root has filled the ball of sphagnum moss. This should be obvious since the root ball will be a mass of fleshy white roots. The more roots there are in the root ball, the greater the chance of the air layering's survival. When you sever the branch, cut it cleanly. Be careful not to handle the root ball too much. Some recommend sawing half way through so that the air layering can be removed in stages.
Place the sphagnum root ball in a sturdy flowerpot and fill it with pure peat. If moss peat is used instead of potting compost or sharp sand, the roots are less apt to break, and that the air layerings therefore have a much greater chance of survival. If a heavy compost, sharp sand, or grit is used its weight will compress, and thus almost certainly damage the brittle roots.
If there are a large number of leaves or branches on the newly-rooted air layering, you should remove some of them in order to reduce transpiration until the air layering is properly established. It is also advisable to stand the newly-potted air layering in a shallow saucer of water so that it can take up adequate moisture during this period. Feeding Vitamin B1 liquid will help to establish the tree quickly, but do not fertilize at this stage as it could damage the very young roots.
Ideally, freshly potted up air layerings should be placed in a damp, humid atmosphere, such as a cool greenhouse, or under mist propagation, as this will encourage roots to form much more quickly. A severed air layering which has been potted up in pure sphagnum moss peat can fill a flowerpot with roots in as little as two to three weeks.
Do not attempt to pot up the freshly rooted air layering in a bonsai pot straight away; it is better to leave the air layering in a flowerpot for at least a year. The air layering should then be grown for a further year in a large seed tray, or in the open ground, to help harden off the roots, thus enabling you to manipulate and plant it in a proper bonsai pot.
In the second year after the air layering has been struck, you can begin to turn it into an attractive bonsai. Once the tree has been potted up, you can train and refine the branches and the overall structure. In time, it will be almost impossible to tell whether the bonsai has been produced from an air layering, or by one of the other more traditional methods, such as from seed, cutting, or grafting.